Field
Cosmos
What the sky is made of, and how long it has been here.
44 entries
The universe has an age
The observable universe is 13.8 billion years old, measured from the cosmic microwave background under ΛCDM.
Nothing outruns light in vacuum
The speed of light in vacuum is invariant, and since 1983 it is defined as exactly 299,792,458 metres per second.
Black holes are real
Spacetime can curve so steeply that light cannot escape. Mergers of such objects have been heard in gravitational waves and imaged in radio light.
The sky still glows from its first light
A nearly isotropic microwave background of 2.725 K fills the sky and is the leftover radiation from recombination.
You are transparent to most of the sun
Hundreds of trillions of solar neutrinos pass through your body every second, and almost none of them hit anything.
The sun is losing millions of tonnes a second
Sunlight is mass. Fusion converts about four million tonnes of the sun into energy every second, and a solar wind carries still more away.
The universe is wider than its age allows
The observable universe is about 93 billion light-years across, even though it is only 13.8 billion years old.
Saturn would float
Saturn’s mean density is about 0.69 grams per cubic centimetre, less than liquid water.
A teaspoon can outweigh a mountain
Neutron-star matter has a density around that of an atomic nucleus. A teaspoon of it would have a mass of order a billion tonnes.
The moon is leaving
Lunar laser ranging shows the Moon receding from Earth by about 3.8 centimetres a year.
The hottest planet’s neighbour keeps ice
Mercury, closest planet to the sun, holds water ice in craters at its poles that sunlight never enters.
There are lakes that are not water
Saturn’s moon Titan has stable lakes and seas of liquid methane and ethane, mapped by Cassini and landed on by Huygens.
An ocean hides under the ice
Saturn’s small moon Enceladus vents salt water from a subsurface ocean, sampled in flight by Cassini.
Giant planets can orbit in days
Planets with the mass of Jupiter orbit some stars in a few days, far inside the distance at which they were expected to form.
Antimatter falls down
Antihydrogen, released in a magnetic trap, accelerates downward under Earth’s gravity, as ordinary hydrogen does.
Gravity keeps the same speed as light
The neutron-star merger GW170817 and its gamma-ray burst arrived within about two seconds after 130 million years of travel. Gravity and light share a speed.
Neutrinos change costume in flight
A neutrino born as one flavor — electron, muon, or tau — can be detected as another. The change requires that neutrinos have mass.
A day on Venus outlasts its year
Venus takes 243 Earth days to spin once relative to the stars, and only 225 Earth days to orbit the sun.
Dead stars keep better time than watches
Pulsars are rotating neutron stars. Their radio beams sweep like a lighthouse, some of them with a stability that rivals atomic clocks.
The bricks of protein fall from the sky
The Murchison meteorite, a carbonaceous chondrite that fell in 1969, contains amino acids that formed in space, not in Earth soil.
Almost none of the universe is made of atoms
In the standard cosmological model, ordinary atomic matter is about five percent of the universe. The rest is dark matter and dark energy.
There are more trees on Earth than stars in the galaxy
A 2015 census estimated about three trillion trees on Earth. The Milky Way contains on the order of one to four hundred billion stars.
The stars you can see are still there
Light from the bright naked-eye stars has usually been traveling for decades to a few thousand years. Those stars live for millions to billions of years. Almost none of them have died while the light was in flight.
A black hole can be less dense than air
If density means mass divided by the volume inside the event horizon, then a sufficiently massive black hole is less dense than water, and a still larger one is less dense than air.
A day on Mercury lasts two years
Mercury orbits the Sun in about 88 Earth days. Sunrise to sunrise on Mercury takes about 176 Earth days, which is two of its years.
Saturn's rings are thinner than a city
The main rings span hundreds of thousands of kilometres and are, over much of their extent, only tens of metres thick.
The Sun is white
Above the atmosphere the Sun's light is white. It looks yellow or red from the ground because air scatters the blue end of its spectrum.
The perfect eclipse is a coincidence of this era
The Sun is about 400 times wider than the Moon and about 400 times farther away, so their discs nearly match. Total solar eclipses are a temporary accident.
A storm on Jupiter is larger than the Earth
The Great Red Spot is an anticyclone observed for more than a century and a half, and for most of that time its long axis has been larger than Earth's diameter.
We have made places colder than space
The cosmic microwave background is 2.7 kelvin. Laboratories have cooled matter to nanokelvin, far below the temperature of deep space.
A comet's tail does not stream behind it
Comet tails point roughly away from the Sun. A comet on its way out of the inner solar system can travel tail-first.
Astronauts come home taller
Without the daily load of gravity the spine lengthens. Astronauts gain a few centimetres in space and lose them again after landing.
Andromeda is wider in the sky than the Moon
The disk of the Andromeda galaxy spans about three degrees, roughly six times the Moon’s width. The eye only picks out the bright centre.
The planets fit between us and the Moon only at the far end
Lined up by their mean diameters, the eight planets span about 393,000 kilometres. The Moon’s distance runs from about 356,000 to about 406,000. They fit when it is far. They do not fit when it is near.
A neutron star’s mountains are centimetres tall
On a neutron star, a bump a few centimetres high is already at the limit of what the crust can hold. The mountains that are likely are smaller.
For a few seconds a burst can outshine its galaxy
A gamma-ray burst pointed at Earth can, briefly, outshine the galaxy that hosts it. The energy is beamed. Most of the universe never sees that particular jet.
Saturn’s north pole is a hexagon
A jet stream around Saturn’s north pole has six straight sides. It is about 30,000 kilometres across, and it has kept the shape since Voyager photographed it in 1980.
A sunset on Mars is blue
The Martian daytime sky is butterscotch. Around the setting sun it turns blue. Fine dust scatters the red light out of the beam and lets the blue continue forward.
A volcano on Mars is the size of a state
Olympus Mons is about 600 kilometres across and about 22 kilometres high, roughly two and a half times the height of Everest. The slope is so gentle that from the base the summit is over the horizon.
Uranus rolls around the Sun on its side
Uranus is tilted by about 98 degrees. Its poles take turns pointing at the Sun, and a season there lasts about twenty-one years.
Europa holds an ocean under the ice
Jupiter’s moon Europa has a shell of ice over a global ocean of liquid water. Magnetic measurements from Galileo are the evidence. The ocean may hold more water than all of Earth’s seas.
Gold is forged when dead stars collide
The collision of two neutron stars, watched in 2017, produced a cloud of the heavy elements. Gold is in that family. How much of Earth's gold came that way is still a share, not a census.
The Moon rang for about an hour after a strike
Apollo seismometers recorded the shock of impacts reverberating in the lunar crust for tens of minutes, sometimes about an hour. The Moon is dry, and dry rock does not damp a wave the way Earth does.
Io is melted by the flexing of its orbit, not by the Sun
Jupiter's innermost large moon is the most volcanic body in the solar system. The heat is tidal. The orbit is kept slightly squashed by the other moons, so Io is kneaded, continuously, as it goes around.